WASHINGTON - The Washington Post Monday reported that conservative billionaires Charles and David Koch have created a political network of "unrivaled complexity built around a maze of groups that cloaks its donors."

The report, conducted with help from the nonpartisan Center for Responsive Politics, said the Koch brothers and fellow donors created a network of politically active groups in 2012 for Republican conservatives that outpaced other independent groups on the right and matched the long-standing national coalition of labor unions that back Democratic candidates.

The report said the "resources and breadth of the organizations made it singular in American politics an operation conducted outside the campaign finance system, employing an array of groups aimed at stopping what its financiers view as government overreach."

Among the beneficiaries of their financing effort is Americans for Prosperity, which is running ads against Sen. Mary Landrieu, D-La., for her support of the Affordable Care Act.

Many of the donations to Americans for Prosperity are undisclosed and the group has refused to say who has financed the current ads against Landrieu.

The Washington Post report said that the Koch groups created a system involving roughly a "dozen limited-liability companies with cryptic, alphabet-soup names such as SLAH LLC and ORRA LLC and entities that dissolved and reappeared under different monikers."